The present invention relates to a communication protocol control apparatus of a data communication apparatus. In a multi-protocol communication control apparatus for selecting an optional communication protocol from a plurality of communication protocols and communicating data between work stations, a conventional apparatus, as described in Japanese Patent Laid-Open No. 1-126044, has means for extracting specific information from header contents of received data and judging whether the extracted information matches the assumed kind of upper layer protocol so as to transfer the received data to the corresponding communication protocol.
The above prior art can use only one kind of transmission line and only one kind of data link control type for the above transmission line. Therefore, a situation where a plurality of data link controllers having lower layer communication protocols is not taken into account even if a plurality of upper layer communication protocols are installed.